Known types of modem tuners are based closely on cable standards, for example in terms of channel spacing, data rates and modulation schemes. FIG. 1 of the accompanying drawings illustrates a typical known modem tuner of the single conversion type for receiving signals selectively from a satellite aerial 1 or a cable input 2. The satellite aerial is connected to a low noise block (LNB) 3, which receives signals, for example, in a band between 3 GHz and 22 GHz The LNB 3 comprises a frequency changer which converts these frequencies into the L band between 900 MHz and 2.2 GHz. Those signals are supplied to an in-door unit (IDU) 4, which converts the signals to be same frequency band as used by a cable distribution system to which the cable input 2 is connected, for example 50 to 900 MHz. The output of the IDU 4 and the cable input 2 are connected to respective inputs of a multiplexer (MUX) 5, which selects which of the signal sources is connected to the input of a tuner 6.
The tuner input is connected to a tracking filter and automatic go control (AGC) circuit 7. The output of the circuit 7 is connected to the signal input of a mixer 8, which receives a local oscillator signal from a local oscillator 9 controlled by a phase locked loop (PLL) synthesiser 10. The tuner 6 is of the single conversion type and the mixer 8 converts the selected channel, which may typically have a bandwidth of 8 MHz, so that it is centred on the intermediate frequency, which is typically 44 MHz. The intermediate frequency signal is supplied via a buffer 11 to a surface acoustic wave (SAW) intermediate frequency bandpass filter 12 which, for the example mentioned above, has a passband of 8 MHz centred on a centre frequency of 44 MHz. The output of the filter 12 is supplied via a buffer 13 to a circuit 14, which performs analogue/digital conversion, demodulation and forward error correction.
FIG. 2 of the accompanying drawings illustrates an alternative type of tuner of the double conversion type. Like reference numerals refer to like parts in FIGS. 1 and 2 and such like parts will not be described again.
The tuner 6 of FIG. 2 comprises an input AGC stage 15 whose output is connected to a first frequency changer comprising a mixer 8a connected to a local oscillator 9a controlled by a PLL synthesiser 10a. The first frequency changer converts the incoming signal selected by the multiplexer 5 to a relatively high intermediate frequency, for example 1.2 GHz. This is filtered by a high intermediate frequency (IF) bandpass filter 16 and supplied to a second frequency changer comprising a mixer 8b connected to a local oscillator 9b controlled by a PLL synthesiser 10b. The second frequency changer converts the high IF signal to a second low intermediate frequency which is typically 44 MHz. The second IF signal is then buffered and filtered before being supplied to the circuit 14 for conversion to and processing in the digital domain.
Known arrangements of the type shown in FIGS. 1 and 2 suffer from various disadvantages. For example, because the signals from the satellite aerial 1 are converted to the same frequency range as the signals from the cable distribution system, interactions may occur between signals from the two sources. For example, if the tuner 6 is set to receive a desired channel from one of the sources and a channel at the same frequency is present from the other source, interactions may occur between the two data streams carried by the signals so that the desired data stream suffers interference.
There are proposed new standards for transmission systems which require that channel bandwidths be variable and increased. Also, different data rates and types of modulation are being proposed in such standards. Existing data rates are typically in the range of 100 kbyte/sec to 45 Mbyte/sec and there are presently proposals for data rates up to 200 Mbytes/sec. Modem tuners of the type shown in FIGS. 1 and 2 are not well-suited to meeting the requirements of the proposed new standards.
For example, higher image rejection in front of or within the mixer 8 or 8a will be required because, with wider channels, the wanted and image channels will be closer together. Also, the bandwidths of the tracking filter of the stage 7 or of the high IF filter 16 will need to be wider in order to cope with the wider channel bandwidths. This makes the image rejection problem worse and also allows more potentially interfering signals to be passed in the case where lower bandwidth channels are to be received. Thus, the potential for intermodulation distortion is increased. Providing bandpass filters of variable bandwidth for the filters 12 and 16 causes substantial problems and is impractical for tracking filters in the circuit 7 of the tuner shown in FIG. 1. Thus, tuners of the type shown in FIGS. 1 and 2 are unlikely to provide satisfactory performance for the proposed new standards,
GB 2 223 900 discloses a television tuner for reception of signals broadcast by satellite. The tuner has an input switch for selecting between signals having two orthogonal linear polarisations and in different frequency ranges. There is an image-tracking filter ahead of the mixer, which performs up-conversion to a high intermediate frequency. The local oscillator incorporates band-switching for receiving different frequency ranges.
U.S. Pat. Nos. 4,162,451 and 4,162,452 disclose a dual-standard television receiver for receiving channels in the UHF band and the VHF bands. Radio frequency switching is provided for selecting between high and low VHF bands. Separate front ends are provided for VHF and UHF reception in some of the disclosed examples. In one of the disclosed examples, switching between VHF and UHF bands is performed upstream of a common mixer, which is provided with two voltage-controlled oscillators for covering the different frequency ranges.